1. Field of the Invention
The present invention relates to a method and apparatus for producing uniform polymeric spheres with membrane permeability varying from semipermeable to impermeable. This invention may be used for encapsulating living cells, or tissue, chemicals or medicines, in these spheres.
2. Description of the Related Art
Spheres have been produced for various medical and biological applications. One of the most important research goals is the transplantation of microspheres which encapsulate islet cells within a semipermeable polymeric membrane to treat diabetes patients. The porosity of the membrane is controlled such that the membrane is permeable to nutrients and to insulin but not the antibody size molecules, i.e,. the membrane acts as an immunoisolation system. This technique for diabetes treatment has been sought as an alternative to insulin injection and to whole-organ pancreas transplants. Studies on rats have shown that the choice of proper materials results in highly biocompatible membranes which maintain appropriate porosity. (A. M. Sun and G. M O'Shea, Microencapsulation of Living Cells--A Long Term Delivery System, Journal of Controlled Release, 2 (1985), 137-141.) (G. M. O'Shea and A. M. Sun, Encapsulation of Rat Islets of Langerhans Prolongs Xenograft Survival in Diabetic Mice, Diabetes, Vol. 35, No. 8, August 1986, 943-946.) As the methods for islet cell isolation become more efficient more high yield means of encapsulation must be developed, (C. R. Ricordi, E. H. Finke and P. E. Lacy, A Method for the Mass Isolation of Islets From the Adult Pig Pancreas, Diabetes, Vol. 35, June 1986, 649-653.) (C. R. Ricordi, P. E. Lacy and D. W. Scharp, Automated Islet Isolation From Human Pancreas, Diabetes, Vol 38, Suppl. 1, January 1989, 140-142.)
This type of treatment may be a solution for hormone deficient patients including insulin-dependent diabetes patients. Other applications for semipermeable microspheres are being developed for the controlled release of drugs and chemicals.
The common technique for producing polymer membranes is through the ionic interaction between polycation and polyanion polymers. By controlling the concentration or molecular weight of the polymer solutions and the number of membrane layers, the permeability of the resulting membrane can be altered. Currently, the most widely used methods for forming the polymer microspheres may be described as follows: the liquid droplets from one polyanion monomer solution are formed by means of a drop generator; these droplets are then immersed either directly into another polycation polymer solution to produce the polymer spheres (J. M. Kendall, M. Chang and T. G. Wang, Fluid and Chemical Dynamics Relating to Encapsulation Technology, AIP Proceedings 197, Third Int'l Colloquium on Drops and Bubbles, Monterey, Calif. 1988.) or first into some chemical solution to harden the droplets, and the hardened droplets then react with another polycation monomer solution to form the desired membrane (A. M. Sun and G. M O'Shea, Microencapsulation of Living Cells--A Long Term Delivery System, Journal of Controlled Release, 2 (1985), 137-141.)
The technique developed by Sun for cell encapsulation has been regarded as one of the most successful methods for microencapsulation of human cells. (A. M. Sun and G. M O'Shea, Microencapsulation of Living Cells--A Long Term Delivery System, Journal of Controlled Release, 2 (1985), 137-141.) (G. M. O'Shea and A. M. Sun, Encapsulation of Rat Islets of Langerhans Prolongs Xenograft Survival in Diabetic Mice, Diabetes, Vol. 35, No. 8, August 1986, 943-946.) The materials used show good biocompatibility with living cells and the process is simple. In general, the cells to be encapsulated are suspended in 1.5% sodium alginate solution (polyanionic). Droplets of this solution are formed by using a syringe pump with a syringe connected to a controlled air jet and are impacted into 1.1% calcium chloride solution to harden. The gel droplets (i.e. calcium alginate) are then immersed into polylysine solution (polycationic) to form the membrane.
Although successful results have been shown, there are some drawbacks with this method. One is the harmful effect of calcium chloride (used to harden the droplets), on the islet cells which results in a lower yield rate. The other is that the impact of the droplets into the calcium chloride deforms the drops and decentralizes the islet cells due to the deceleration, the cells are pushed onto the boundary of the droplet, and usually a bump will form on the hardened droplet. The imperfect surface of the droplet provides the chance for fibroblastic growth to occur at the point of surface discontinuity and induces an inflammatory reaction. Another drawback is the possibility of limited lifetime of this membrane inside of a body due to the fact that the membrane is biodegradable in time. It is obvious that these obstacles must be overcome before mass production can be achieved.
To address the problem of toxic effect of calcium chloride on the islet cells, Kendall, Chang and Wang suggest that the intermediate step of the gel hardening process may be omitted.(J. M. Kendall, M. Chang and T. G. Wang, Fluid and Chemical Dynamics Relating to Encapsulation Technology, AIP Proceedings 197, Third Int'l Colloquium on Drops and Bubbles, Monterey, Calif. 1988.) Instead of directing the droplets into the calcium chloride solution, the droplets are introduced into a chitosan solution. A polymer membrane forms when the droplet penetrates the liquid surface. As the chitosan solution is not harmful to the living cells, there will be no damage done to the encapsulant. However, further investigation reveals that new difficulties arise when the proposed method is applied. First of all, the properties of the calcium chloride solution are totally different from that of the chitosan solution. A 0.2% chitosan solution is about ten times as viscous as the water-like calcium chloride solution and the surface tension may also be changed. The experiments show that it is difficult for the submillimeter-size droplets to penetrate the liquid surface without excess deformation. Higher velocity is required and the penetration time is increased. The high impact velocity results in highly distorted microspheres, while the increased penetration time results in the collision of the second droplet on the penetrating one and the droplets stick together.
As such, a need exists in the industry to provide an apparatus and method to produce uniformly concentric semipermeable microspheres that can encapsulate living cells or tissue. More particularly a need exists in the industry to provide a method to encapsulate living cells and tissue wherein the tissue is encapsulated in a first monomer prior to polymerization so that a broader range of polymers may be used.